Alkaline electrochemical cells employing a zinc anode have become commercially important as a relatively high rate source of electrical energy. The alkaline electrolyte, which is usually concentrated aqueous potassium hydroxide, is a principal factor contributing to the ability of these cells to deliver high rates of electrical energy compared to the older Leclanche cells which utilize zinc chloride and/or ammonium chloride electrolytes. However, as is so often the case with technological advances, the presence of alkali in an electrochemical cell is a mixed blessing. For instance, alkali strongly promotes the reaction of water with zinc causing corrosion. Unless means are employed to control this reaction, the shelf life of alkaline zinc cells would be unacceptably short. In addition, since hydrogen gas is released in the reaction between alkali and zinc, there may be a danger of cell disassembly.
In commercial alkaline zinc cells, the reaction between zinc and alkali has been controlled or reduced to an acceptable rate by the addition of mercury to the cell. Unfortunately, it has recently become apparent that the introduction of mercury into the environment may be a hazard to human health as well as to other forms of life. While individual cells contain only a small amount of mercury, the very large number of zinc alkaline cells marketed today could cause significant quantities of mercury to enter the environment upon disposal of such cells.
U.S. Pat. No. 3,847,699 disclosed an alkaline zinc-manganese dioxide cell in which the proportion of mercury needed to achieve acceptable storage life can be reduced by the addition of small amounts of an ethylene oxide polymer.
U.S. Pat. No. 4,500,614 discloses an alkaline cell having an anode prepared by amalgamating an alloy powder made of zinc and at least two metals selected from the group consisting of gallium, indium and thallium. The metals are incorporated in the cell to reduce the amount of mercury required to prevent corrosion of the zinc in an alkaline electrolyte which causes generation of hydrogen gas and subsequent leakage of the electrolyte.
German Patent 1,086,309 discloses an alkaline zinc cell in which an indium compound is added to the electrolyte and/or indium metal is alloyed with refined zinc so as to protect the zinc against corrosion in an acidic, neutral or alkaline electrolyte.
Japanese Publication No. 1958-3204, published Apr. 26, 1958, recites that the addition of 0.0001% to 2.0% indium can be added to pure zinc base alloy containing one, two, or more of the metallic elements Fe, Cd, Cr, Pb, Ca, Hg, Bi, Sb, Al, Ag, Mg, Si, Ni, Mn, etc., to form a zinc alloy which has a high corrosion resistance and which is suitable for use in primary cells.
Japanese unexamined patent application 01-307161 is directed to a mercury-free alkaline cell wherein the negative electrode's collector is coated with indium and/or lead in which the coating can be supplied by any method such as electroplating.
As seen from the above, manufacturers of alkaline batteries have invested substantial amounts of time and money in the effort to develop mercury-free batteries. The first batteries to incorporate mercury-free constructions were the AA, C and D standard alkaline batteries. These batteries have historically used the largest quantities of mercury per cell and are the most common sizes and type purchased by consumers. Therefore, removing mercury from these batteries was the best way to significantly reduce the quantity of mercury that is currently entering the general waste stream when used alkaline cells are thrown away.
In addition to developing mercury-free standard alkaline batteries, battery manufacturers have also sought to develop miniature zinc-air battery constructions that are free of mercury. While a layman may believe that the technology used to produce mercury-free standard alkaline batteries can be used to produce mercury-free miniature zinc-air batteries, the manufacturers of miniature zinc-air batteries have found that additional technology had to be developed before mercury-free miniature zinc-air cells could be manufactured on a commercial basis. The construction of a miniature zinc-air battery is substantially different from the construction of a standard alkaline battery. These differences in construction have forced battery manufacturers to develop processes and techniques that pertain only to the miniature zinc-air cells.
It is an object of the present invention to provide a miniature zinc-air cell which has a reduced amount of mercury of less than 6%, preferably less than 3%, in the zinc-containing electrode and most preferably having a mercury-free zinc-containing electrode.
It is another object of the present invention to provide a miniature zinc-air cell with a zinc-containing electrode that is substantially free or completely free of mercury and wherein the surface of the electrode cup contacting the zinc-containing electrode has an undercoat of copper and a top coat of indium.
It is another object of the present invention to provide a method for producing an indium coated cup for housing a zinc-containing electrode of a miniature zinc-air cell.
These and other objects of the invention will be apparent from the following description.